The present invention relates to antitumor antibiotic herein designated as kedarcidin, to its production by Streptoalloteichus sp. nov. Strain L585-6, to pharmaceutical compositions containing the antibiotic and to method of inhibiting tumor growth by said antibiotic. The invention also relates to the kedarcidin-producing microorganism Streptoalloteichus sp. nov. Strain L585-6, ATCC 53650.